Perlite ore is a naturally occurring volcanic mineral consisting primarily of silica, alumina and a small amount of water. Exposure of perlite ore to temperatures in the range of 1700 to 2100.degree. F. softens the mineral and causes the water to expand to form a light cellular mineral particle. Due to the low density and low thermal conductivity of expanded perlite, it has found utility as a thermal insulator. Expanded perlite has been used in its particulate form, for example, as loose-fill insulation. A co-pending application, Ser. No. 875,632, filed Feb. 6, 1978, now U.S. Pat. No. 4,212,755, discloses a loose-fill insulation of expanded perlite particles coated with a material rendering the particles slightly tacky.
It is also known to use expanded perlite as a component of an insulating board. Conventional perlite board is made from an aqueous slurry of fibers, sizing and expanded perlite, that is formed into a board by a Fourdrinier process and subsequently dried. Because the mixture of fiber and expanded perlite is an efficient thermal insulator, it requires a great amount of energy to remove the water component of the slurry from the core of a thick, conventionally formed perlite board. Therefore, it has been necessary to laminate two relatively thin perlite boards to get a product thickness greater than about 1 1/4 inch. The lamination of such boards significantly increases the cost of production and renders the product susceptible to potential problems with delamination of the layers.
The present invention overcomes this difficulty of the prior art by producing thick, expanded perlite without resorting to either an energy inefficient Fourdrinier process or lamination of thin boards. In addition, the product may be formulated in such a manner that it is flexible.